What Are "Digital Surrogates" Good for, and What Don't They Tell Us?
Since the rise of the Internet and the almost complete triumph of digital photography over chemicals and film, a popular way to access some form of rare manuscript and early printed evidence is the "digital facsimile." These "surrogates" for the real thing can supply amazingly important tools for the scholar who cannot afford to travel to see the real thing. They also save these precious survivors from the inevitable damage that would be caused by continuous handling. In fact, you have to make a pretty good case to a rare book or MS curator to be allowed to get your hands on the rarest of them. "Use the digital facsimile," they'll tell you.
Digital facsimiles also provide scholars with the ability to magnify the documents they study. This can be an enormous help when one is trying to decipher manuscript hands, or to recover partially erased or cancelled (scratched out) passages. It also can enable side-by-side comparison of leaves from the same document that are located dozens or hundreds of leaves apart in the same binding. Those are the advantages.
Digital facsimiles inherently misrepresent the document's size, tending to shrink everything to fit the monitor's shape, or allowing the image to spill over into the invisible space around the monitor, requiring us to scroll around to see the hidden parts of the leaf. Digital facsimiles, even if color-corrected by the most meticulous photographers, misrepresent the document's colors, including ink colors and manuscript illuminations or hand-colored print engravings. Color is important evidence in codicology, the analysis of "codecies" or bound documents (AKA "books"). Facsimiles also eliminate tactile, olfactory, and other sources of sensory evidence that trained investigators can analyze productively. From the composition of the substrate (paper composition; parchment source identification) to foreign matter deposited on the page (candle wax, spiders and other insects, pollen and other vegetable matter), the presence of the physical artifact provides evidence of provenance (where the document was made and where it has been) and readers' use of it, all of which is inaccessible through a digital surrogate.
Use the digital surrogate for the purposes in which it excels. Otherwise, use the real thing.